1. Field of the Invention
The present invention relates to a mobile communications system, and more particularly, to an apparatus for performing initial synchronization and frame synchronization in a mobile communications system and method thereof.
2. Discussion of the Related Art
Generally, a cell search process by a user equipment (hereinafter abbreviated UE) in a TD-SCDMA system is a process for obtaining downlink synchronization code (SYNC-DL), basic midamble code, scrambling code, frame synchronization, BCH (broadcast channel) information, chip timing information and the like. And, the cell search process is carried out by the following 4-step procedures in general.
FIG. 1 is a flowchart of a cell search process according to a related art.
1) Initial Synchronization Search Procedure (Search for DwPTS)
First of all, a UE performs initial synchronization in DwPTS (downlink pilot time slot) using thirty-two available SYNC-DL codes without information for a cell to be searched for. Through this procedure, the UE should find the SYNC-DL code used in the cell among the thirty-two available SYNC-DL codes and a timing synchronization point.
2) Basic Midamble and Scrambling Code Identification
Since the DwPTS code found by the above initial synchronization search procedure corresponds to one code group having four basic midambles, the UE searches for a basic midamble in use within the cell from the four basic midambles. Because the basic midamble is associated with a scrambling code, the scrambling code can be found if the basic midamble is obtained.
3) Frame Synchronization Acquisition (Control Multi-Frame Synchronization)
The UE searches for MIB (master information blocks) of BCH multi-frame by QPSK phase modulation of DwPTS for P_CCPCH midamble.
4) BCH Information Acquisition (Read the BCH)
The UE acquires BCH information by reading the BCH using the searched MIB.
A frame synchronization acquisition process according to a related art is explained in detail as follows.                First of all, since BCH in TD-SCDMA system has 20 ms transmission time interval (hereinafter abbreviated as TTI), a UE has to know whether or not BCH exists in a next frame and a position where the 20 ms TTI starts.        
In order to inform the UE of a start position of BCH, a base station performs phase modulation based on a phase of a first timeslot midamble in case of modulating DwPCH. And, a value of the phase modulation is identically sustained during one subframe.
Four phase modulation values obtained during four consecutive subframes referred to as a ‘Phase Quadruple’ and there are two kinds of phase quadruples, S1 and S2. Table 1 shows meanings of the two kinds of phase quadruples. In Table 1, P-CCPCH (primary-common control physical channel) is a physical channel to which a transport channel BCH is mapped.
CasePhase QuadrupleMeaningS1135°, 45°, 225°, 135°P-CCPCH exists in next foursubframesS2315°, 225°, 315°, 45°P-CCPCH does not exist in nextfour subframes
In the related art, ‘Phase Quadruple’ is detected during four subframes, and it is then decided whether the detected phase quadruple corresponds to S1. If it is decided as S1, it is decided that there exists P-CCPCH from a next subframe. Reception of BCH transmitted via P-CCPCH is then initiated.
In particular, a process for detecting the phase quadruple includes a procedure for detecting a phase modulation value of a downlink synchronization code for each subframe, which consists of the following steps.
First of all, a phase value of a received midamble code and a phase value of a downlink synchronization code are detected.
Subsequently, correlation between the detected phase values of the two codes is obtained. By taking ‘arctangent’ on the obtained correlation, a modulated phase of the downlink synchronization code is then found.
Thereafter, the phase quadruple, which is the phase value modulated during the four subframes obtained in the above manner, is compared to the phase value S1 or S2 in Table 1.
As a result of the comparison, if the obtained phase quadruple corresponds to the S1, BCH is received in next four subframes. If the obtained phase quadruple does not correspond to the S1, it is decided that BCH does not exist in next four subframes. So, the process for detecting the phase quadruple is repeatedly executed.
Yet, in case that the modulated phase value of the downlink synchronization code is detected using the ‘arctangent according to the related art, the arctangent operation is replaced by a table mapping using a lookup table for efficient implementation in general. So, since sufficient amount of table values need to be stored in a prescribed memory for accurate operations, a memory capacity should be excessively increased.
And, since the modulated phase value of the downlink code can be distorted by a frequency offset as well, it is difficult to calculate an accurate phase modulation value by the related art method.
The first procedure of initial synchronization search in the cell search process may have an effect on power consumption, automatic frequency control (AFC), synchronization tracker performance and the like. In addition, because the initial synchronization search should be repeatedly executed many times, it is required to satisfy excessive calculations and memory.
Generally, a downlink synchronization code (SYNC-DL) currently used by a current cell is detected through correlation between a signal received from a base station and a reference signal. In the 3GPP TDD LCR (low chip rate) system, a method of performing initial synchronization from a received signal includes one of the following schemes.
First of all, there is a method using a maximum value of correlation. In particular, initial synchronization is performed by finding a maximum value using correlation from one received subframe. So, the method can be easily implemented.
Yet, since a level of a received signal is rapidly varied in a multi-path fading channel environment with low SNR, it is difficult to improve performance of temporal correlation. Moreover, in case of reducing the sampling occurrences of received signals, a timing error increases.
Secondly, there is another method using an average calculated by accumulating correlation values through several subframes. In particular, correlation between a signal received from a base station and a downlink synchronization code (SYNC-DL) is obtained. The correlations are accumulated through several subframes and averages are then obtained. And, the initialization synchronization is performed by searching the averages for a biggest value.
If a maximum value is found by obtaining averages resulting from accumulating correlations through several subframes, it is able to obtain a reliable timing correlation by reducing a level variation of a received signal due to an effect of high fading frequency. Yet, since the correlations for thirty-two codes received through several subframes should be stored each time, the memory increase is inevitable. And, in case of over-sampling the received signals and a comparator for finding sequentially a maximum correlation is implemented, comparisons and storages should be repeatedly conducted as many as [6400×32×over-sampling].
Moreover, since it is unable to set a number of subframes to an infinite number, it is restricted to a prescribed number. So, if an average is taken despite that error values of correlations considerably distorted by noise are accumulated, degradation of performance is inevitable.
Thirdly, there is another method using tracking a synchronization code having a high occurrence number and a synchronization point thereof for several subframes. Considering probability decided through several subframes, an occurrence number having a maximum correlation in an original code is high. So, erroneous correlations are removed using the occurrence frequency and an average is obtained with remaining strong timing candidates.
If only an index of a code having a biggest correlation value for one subframe and the correlations value are stored, it is able to considerably reduce a memory size necessary for one subframe. However, sufficient amount of subframes should be observed in order to track a code having a high occurrence number for several subframes and a synchronization point. If less subframes are observed, accuracy is lowered.
Moreover, if a channel environment is poor, it is unable to find an accurate downlink synchronization code. So, more subframes need to be observed. If not, the accuracy is considerably lowered.